What Causes Brown Colored Mustache Growth?

Hi All, i am 26 years old guy. I had black mustache for 6 years since i was 20 ,few weeks back i noticed my mustache has turned brown on my right side around area of 1 mm. Now the less hair is growing in that area . Also it is growing brown not black. i have black mustache. recently i observed the area is increasing. At first it was just below the corner of right nose. now it is spreading towards middle of upper lips
There is no redness no itiching, i dont feel any kind of problem in that area other than hair lost. can you tell me what might be the reason and how can i stop that from spreading.
Thanks